A category of power supplies known as switching power supplies date back several decades and are currently heavily utilized in the electronics industry. Switching power supplies are commonly found in many types of electronic equipment such as industrial machinery, automotive electronics, computers and servers, mobile consumer electronics (mobile phones, tablets, etc.), battery chargers for mobile electronics, and low cost/light weight items such as wireless headsets and key chain flashlights. Many applications include switching power supplies for portable, battery powered devices where an initial voltage is stepped down to a reduced voltage for part of the device, such as integrated circuits that operate at fairly low DC levels. Switching supplies are popular because these powers supplies can be arranged to be light weight, low cost, and are highly efficient in the conversion of the voltage and current levels of electric power when compared to the prior approaches using non-switching power supplies such as linear power supplies.
High efficiency is achieved in switching power supplies by using high speed low loss switches such as MOSFET transistors to transfer energy from the input power source (a battery, for example) to the electronic equipment being powered (the load) only when needed, so as to maintain the voltage and current levels required by the load.
Switching power supplies that perform conversion from a direct current (DC) input (such as a battery) that supplies electric energy within a specific voltage and current range to a different DC voltage and current range required by the load are known as “DC-DC” converters. Many modern prior known approach DC-DC converters are able to achieve efficiencies near or above 90% by employing zero voltage transition (ZVT) functionality. The ZVT technique was developed by Hua, et. al. and described in a paper published in 1994 (“Novel Zero-Voltage-Transition PWM Converters,” G. Hua, C.-S. Leu, Y. Jiang, and F. C. Lee, IEEE Trans. Power Electron., Vol. 9, No. 2, pp. 213-219, Mar. 1994). The use of the ZVT function in prior known approach DC-DC converters reduces energy loss that would otherwise occur due to switching and has the additional benefit of reducing voltage stress on primary power switches of DC-DC converters. Reduction in voltage stress on a switch allows the switch to have a lower voltage tolerance rating and, therefore, potentially the switch can be made smaller and less costly.
The ZVT circuitry employed by prior DC-DC converters introduces additional switches and corresponding additional energy loss and voltage stress on switching elements. However, the impact of energy loss and voltage stress of the ZVT function is much less significant than the overall performance improvements to switching converters that employ ZVT functionality. Further improvements to reduce energy loss and voltage stress of the ZVT function are still needed and these improvements will permit improvement of electronic equipment in multiple ways including increased battery life, lower cost of operation, and improved heat management.
To better illustrate the shortcomings of the prior known ZVT approaches, circuit 10 of FIG. 1 illustrates a typical ZVT DC-DC converter arranged in a circuit topology known as a buck converter. Buck DC-DC converters provide an output voltage at a lower voltage than an input voltage. Other types of DC-DC converters that can benefit from the use of ZVT switching include, but are not limited to, boost converters that increase voltage to the load to a voltage greater than the input voltage, and buck-boost DC-DC converters that dynamically transition between the buck and boost functions to adapt to various input voltage levels that could be either greater or less than the output voltage required by the load.
FIG. 1 illustrates in a simplified circuit diagram the switching elements, key passive components, and key parasitic elements of a ZVT DC-DC buck power converter 10. Omitted from FIG. 1 for increasing the simplicity of explanation are minor components, minor parasitic elements, the circuits for monitoring output voltage, and the control circuit for controlling the switch timing that are utilized in typical prior ZVT DC-DC buck power converters.
Circuit 10 shown in FIG. 1 contains two primary power switches, S1 and S2, that in conjunction with the output inductor Lo and capacitor Co are used to perform the primary function of the buck converter of supplying energy to the load (represented as a resistive load, Ro) at an output voltage level Vo that is a reduced voltage from the DC input voltage, Vin. Vin represents both the external element that is the source of input voltage (such as a battery or another power supply) to the ZVT power converter and the voltage level across the positive and negative terminals of the Vin input voltage source. Auxiliary switches Sa1 and Sa2 and the auxiliary inductor La are the added components (added to the prior switching converter topology) that are used to accomplish the ZVT functionality. A primary parasitic inductance that contributes to voltage stress on switch S2 is represented in FIG. 1 by inductor Lbyp. The source terminal of S1, the drain terminal of S2 and one terminal of each inductor La and Lo are coupled as illustrated in FIG. 1 to a common node known as the switch node and labeled Switch Node in FIG. 1. The first auxiliary switch Sa1, the second auxiliary switch Sa2, and the auxiliary inductor La are coupled together at an auxiliary node labeled Aux Node. All four switches in the non-limiting, illustrative example buck converter 10 of FIG. 1 (S1, S2, Sa1, and Sa2) are shown implemented as enhancement mode n-channel MOSFETs. Drain to source parasitic capacitances of switches S1 and S2 are important to the circuit description and are illustrated in FIG. 1 as Cds1 and Cds2 respectively. The intrinsic body diode of MOSFET switches is also shown connected between source and drain for all switches (S1, S2, Sa1, and SA2) of FIG. 1.
While enhancement mode n-channel MOSFETs are commonly used as switches in prior DC-DC converters as shown in the example in FIG. 1, other types of transistor switches as well as diode switches in some cases have been employed and can be used to form the buck converter 10, or to form other types of switching power converters.
Circuit 10 illustrated in FIG. 1 accomplishes the primary buck converter function of supplying a reduced voltage to the load (voltage across resistor Ro) by alternatively switching between two primary states. In one of the primary states (defined by switch S1 closed and switch S2 open, which means switch S1 is a transistor that is turned on, while switch S2 is a transistor that is turned off), the input voltage source (Vin) supplies energy to the load, and energy to maintain or increase magnetic energy is also stored in inductor Lo. In the other primary state (defined by switch S1 open and switch S2 closed, which means that switch S1 is a transistor that is turned off, while switch S2 is a transistor that is turned on), current flow from the input (Vin) is blocked, and the magnetic energy stored in inductor Lo is converted to electric energy and supplies energy to the load (resistor Ro). The voltage across the load Ro is maintained in a pre-defined range by varying the relative amount of time the circuit spends in each primary state. Converters that alternate between the two states described above are sometimes described as pulse width modulated (PWM) switching converters because the output Vo is proportional to the input voltage Vin, multiplied by the duty cycle of switch S1 (a ratio of the on time of switch S1 to the total cycle period). Typically, prior known buck converters cycle between these states fairly rapidly (often at hundreds of KHz to 1 MHz and above). In addition to the two primary states, there are brief dead times during the transitions between the two primary states. During the dead times, switches S1 and S2 are simultaneously open, that is the transistors implementing switches S1 and S2 are simultaneously turned off. Dead times are used to insure there is not a high current path across the input voltage source (Vin) directly to ground, which could occur when both S1, and S2, are closed. Prior known approach PWM switching power supplies employ two dead times during each cycle of operation: a first dead time occurs when switch S1 opens and ends when switch S2 closes; and another second dead time occurs when switch S2 opens and ends when switch S1 closes. The ZVT function operates in a small amount of time that begins prior to the beginning of the second dead time with S2 opening, and the ZVT function ends a small amount of time after the second dead time ends with switch S1 closing. The ZVT function does not operate in the first dead time of the buck converter cycle described above (the time between switch S1 opening and S2 closing).
FIG. 2 illustrates in a timing diagram the sequence of switch transition events to operate ZVT functionality for prior known approach ZVT DC-DC buck converters. In FIG. 2 the switching events are labeled t0, t1, t3, and t4. (It should be noted that there is no event labeled t2 in FIG. 2 for increasing simplicity of explanation when comparing the switching event sequence of prior known approach ZVT DC-DC buck converters with the switching event sequence of example arrangements of the present application.) The dead time described above during the time interval between switch S2 opening and switch S1 closing begins at event t1 and ends at event t3 illustrated in FIG. 2.
The open and closed states of each of the four switches (S1, S2, Sa1, and Sa2) illustrated in FIG. 1 are represented in FIG. 2 by the voltage applied to the switch gates (Vg1, Vg2, Vga1, and Vga2 respectively) and shown in four graphs, graph 201 illustrates the voltage on the gate of switch S1, graph 202 illustrates the voltage on the gate of switch S2, graph 203 illustrates the voltage on the gate of switch S3, and graph 204 illustrates the voltage on the gate of switch S4. A voltage annotated as Von applied to a switch gate indicates the switch is closed, and a voltage annotated as Voff indicates the switch is open. The purpose of FIG. 2 is to illustrate of the sequence of switching events, and does not illustrate specific voltage levels, waveform shapes, and time increments.
ZVT functionality for prior known approaches begins at event labeled t0 in FIG. 2 with switch Sa1 turning on as shown in graph 203. In the time leading up to event t0 switch S2 has been closed and switches S1 and Sa2 have been open for a significant portion of the current buck converter cycle. Time progresses to event t1 illustrated in FIG. 2 when switch S2 opens as shown in graph 202. At the next event, t3, switches S1 and Sa2 close as shown in both graphs 201, 204. Switch Sa1 opens at tome t3, as shown in graph 203, and after a short delay to provide the dead time, Sa2 closes just after event t3 as shown in graph 204. At event t4, Sa2 opens as shown in graph 204 to complete ZVT functionality for the current cycle of the buck converter.
The typical prior ZVT buck converter circuit illustrated by circuit 10 in FIG. 1 accomplishes ZVT when the primary power switch S1 transitions from open to closed (S1 turn on as shown in graph 201) at event labeled t3 illustrated in FIG. 2 with zero or near zero volts across it. For the circuit 10 to reach a condition with zero or near zero volts across switch S1 prior to S1 turning on or closing, an L-C resonant circuit is used to increase the voltage at the source terminal of switch S1 (coupled to the node “switch node” in FIG. 1) until approximately equivalent to the voltage at the drain terminal of S1, which is coupled to and approximately equivalent to the input voltage, Vin. The above L-C resonant circuit includes the inductor La and the parallel combination of capacitances Cds1 and Cds2 (the drain to source parasitic capacitances of the switches S1 and S2 respectively) and is referenced herein as the “ZVT resonant circuit.” The ZVT resonant circuit is a portion of circuit 10. For prior known approaches, the ZVT resonant circuit resonates only when switch Sa1 is closed and switches S1, S2, and Sa2 are open, which is the time span between events t1 and t3 in FIG. 2. The time span between events t1 and t3 for typical prior known approaches is equivalent to one-quarter cycle of the natural resonant frequency of the ZVT resonant circuit.
While prior known DC-DC converters incorporating the ZVT function typically have lower energy loss and lower voltage stress on transistor switches when compared to the earlier prior DC-DC converters formed without the ZVT function, the ZVT function itself introduces energy loss and voltage stress.
There are two key contributors to energy loss of prior approach ZVT functions that are reduced in the arrangements of the present application. First, energy is lost when auxiliary switch Sa1 turns off when conducting peak current as it transitions through the MOSFET linear region. The second key contribution to energy loss during the ZVT operation is the sum of conduction losses through switches Sa1, Sa2, and S1 and inductor La.
The most significant impact of voltage stress resulting from the ZVT function is on the voltage tolerance required for switch S2 and, therefore, this impacts S2 transistor size and potential cost. The voltage stress on switch S2 is the result of switch Sa1 turning off with peak current flowing through it, causing a voltage spike across switch S2 induced by the parasitic inductance, Lbyp. In addition, there is a voltage spike across Sa1 when it turns off with current flowing through it, due to ringing with parasitic inductances. However, sizing Sa1 for higher voltage tolerance is not a significant impact to potential converter cost, since Sa1 is already a small transistor when compared to the primary power transistors, S1 and S2.
Improvements are thus desirable in the performance and efficiency of ZVT converters. Improvements that reduce the power losses of the ZVT converter over prior known approaches and that reduce voltage stress, enabling the use of smaller and lower cost components to implement the ZVT converter, are of particular importance.